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Abstract:

A battery charger includes a power source for supplying a primary charge
current to a first battery, and a charge manager for charging a second
battery. The charge manager is coupled to the power source and is
configured to charge the second battery with a secondary charge current
in accordance with a continuous comparison between a predefined maximum
current limit and a total current drawn from the power source.

Claims:

1. A battery charger comprising:a power source for supplying a primary
charge current to a first battery; anda charge manager for charging a
second battery, the charge manager being coupled to the power source and
being configured to charge the second battery with a secondary charge
current in accordance with a continuous comparison between a predefined
maximum current limit and a total current drawn from the power source.

2. The battery charger according to claim 1, wherein the charge manager
comprises a controlled current/voltage source for supplying the secondary
charge current, and a charge manager controller coupled to the controlled
source, the charge manager controller being configured to control a
magnitude of the secondary charge current by outputting to a control
input of the controlled source an analog difference signal proportional
to a difference between a magnitude of the maximum current limit and the
total drawn current.

3. The battery charger according to claim 2, wherein the charge manager
controller comprises a voltage reference, a current monitor coupled to
the power source, and a differential amplifier coupled to the voltage
reference and the current monitor, the voltage reference being
proportional to the maximum current limit, the current monitor being
configured to output an analog voltage proportional to the total drawn
current, the differential amplifier being configured to output the analog
difference signal, the analog difference signal being proportional to a
difference between the voltage reference and the analog voltage.

4. The battery charger according to claim 3, wherein the voltage reference
is configured to limit the primary charge current to less than a maximum
current limit of the power source.

5. The battery charger according to claim 1, wherein the charge manager
and the power supply are disposed within a common charger housing, the
charger housing comprising comprises a first indicator for indicating a
charge state of the first battery, and a second indicator for indicating
a charge state of the second battery.

6. The battery charger according to claim 5, wherein the first battery is
disposed within a handheld computing device, the charger housing is
configured to capture the second battery externally thereto, and further
comprises a battery interface for electrically interfacing with the
second battery, and a charger output for supplying the primary charge
current to the handheld computing device.

7. The battery charger according to claim 1, wherein the charge manager is
configured such that, while the handheld computing device charges the
first battery with a continuously decreasing portion of the primary
charge current, the controlled source charges the second battery with a
continuously increasing portion of the secondary charge current, the
total of the decreasing portion and the increasing portion being
substantially equal to the maximum current limit.

8. The battery charger according to claim 7, wherein the charge manager is
configured to charge the second battery with a second major constant
portion of the secondary charge current, after a current drawn by the
second battery during the continuously increasing charging step exceeds a
maximum current threshold, the second major constant portion being
substantially equal to the maximum current threshold.

9. The battery charger according to claim 7, wherein the charge manager is
configured to charge the second battery with a minor constant portion of
the secondary charge current prior to the continuously increasing
charging step, while the handheld computing device charges the first
battery with a first major constant portion of the primary charge
current, the total of the first major portion and the minor portion being
substantially equal to the maximum current limit.

10. The battery charger according to claim 8, wherein the charge manager
is configured to charge the second battery with a minor constant portion
of the secondary charge current prior to the continuously increasing
charging step, while the handheld computing device charges the first
battery with a first major constant portion of the primary charge
current, the total of the first major portion and the minor portion being
substantially equal to the maximum current limit.

11. A method of simultaneously charging batteries from a power source, the
method comprising supplying a primary charge current from the power
source to a first of the batteries, while charging a second of the
batteries with a secondary charge current from the power source in
accordance with a continuous comparison between a predefined maximum
current limit and a total current drawn from the power source.

12. The method according to claim 11, wherein the second battery charging
step comprises controlling a magnitude of the secondary charge current by
outputting to a control input of a controlled current/voltage source an
analog difference signal proportional to a difference between a magnitude
of the maximum current limit and the total drawn current.

13. The method according to claim 12, wherein the analog difference signal
is determined in accordance with a difference between a voltage reference
and an analog voltage signal, the voltage reference being proportional to
the maximum current limit, the analog voltage signal being proportional
to the total drawn current.

14. The method according to claim 13, wherein the voltage reference limits
the primary charge current to less than a maximum current limit of the
power source.

15. The method according to claim 11, wherein the first battery is
disposed within a handheld computing device, and the second battery
charging step comprises charging the second battery with a continuously
increasing portion of the secondary charge current while the handheld
computing device charges the first battery with a continuously decreasing
portion of the primary charge current, the total of the decreasing
portion and the increasing portion being substantially equal to the
maximum current limit.

16. The method according to claim 15, wherein the second battery charging
step further comprises charging the second battery with a second major
constant portion of the secondary charge current, after a current drawn
by the second battery during the continuously increasing charging step
exceeds a maximum current threshold, the second major constant portion
being substantially equal to the maximum current threshold.

17. The method according to claim 15, wherein the second battery charging
step further comprises charging the second battery with a minor constant
portion of the secondary charge current prior to the continuously
increasing charging step, while the handheld computing device charges the
first battery with a first major constant portion of the primary charge
current, the total of the first major portion and the minor portion being
substantially equal to the maximum current limit.

18. The method according to claim 16, wherein the second battery charging
step further comprises charging the second battery with a minor constant
portion of the secondary charge current prior to the continuously
increasing charging step, while the handheld computing device charges the
first battery with a first major constant portion of the primary charge
current, the total of the first major portion and the minor portion being
substantially equal to the maximum current limit.

Description:

FIELD OF THE INVENTION

[0001]The invention described herein relates to a battery charger. In
particular, this invention relates to a method and apparatus for
simultaneously charging multiple batteries.

BACKGROUND OF THE INVENTION

[0002]It is not uncommon for operators of battery-powered portable
communications devices, such as wireless telephones, personal data
assistants, wireless pagers, and portable computers, to carry one or more
spare batteries to extend the operational time of the device.
External-type chargers are popular because they allow the operator to
continue using the communications device while the other battery charges.
However, typically portable chargers only allow a single battery to be
charged at a time. Such "single-capacity" chargers have the obvious
disadvantage of requiring lengthy recharge periods, particularly where
the operator has multiple spare batteries. Therefore, attempts have been
made to provide a battery charger that allows the operator to charge more
than one battery at a time.

[0003]For instance, Murakami (U.S. Pat. No. 6,794,851) describes an
external battery charger that allows a mobile phone to charge the battery
that is installed in the mobile phone, while simultaneously charging a
second battery that is external to the mobile phone. To apportion the
charge current between the batteries, Murakami makes use of the fact that
the resistance of a battery increases as the battery becomes charged.

[0004]Frame (U.S. Pat. No. 6,005,358) describes a portable computer
docking station that supplies power to a portable computer, and includes
a first charge circuit for charging the battery installed in the portable
computer, and a second charge circuit for charging the battery installed
in the docking station. Each charge circuit measures the current that is
drawn from the power supply, and adjusts the charge current that they
supply to their respective batteries based on these measurements.

[0005]None of these solutions make optimum use of the maximum charge
current that is available from the power supply. As a result, charge
times are unnecessarily long.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]The invention will now be described in detail, by way of example
only, with reference to the accompanying drawings, in which:

[0007]FIG. 1 is an overview schematic diagram of the battery charger;

[0008]FIG. 2 is a flowchart that depicts, by way of overview, the charging
method implemented by the battery charger;

[0009]FIGS. 3A to 3C together comprise a flowchart that depicts, in
detail, the charging method implemented by the battery charger;

[0010]FIG. 4 is a diagram that depicts the resulting charge profiles for
the (first) battery of the handheld computing device, and the (second)
battery that is held by the battery charger; and

[0011]FIG. 5 is a detailed schematic diagram that depicts a preferred
implementation of the battery charger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0012]By way of overview, the invention described herein relates to a
battery charger and a method for simultaneously charging multiple
batteries.

[0013]As will be described in further detail below, the battery charger
includes a power source for supplying a primary charge current to a first
of the batteries, and a charge manager for charging a second of the
batteries. The charge manager is coupled to the power source and is
configured to charge the second battery with a secondary charge current
in accordance with a continuous comparison between a predefined maximum
current limit and a total current drawn from the power source.

[0014]In a preferred implementation, the charge manager comprises a
controlled current/voltage source which supplies the secondary charge
current, and a charge manager controller coupled to the controlled
source. The charge manager controller is configured to control the
magnitude of the secondary charge current by outputting to the control
input of the controlled source an analog difference signal that is
proportional to the difference between the magnitude of the maximum
current limit and the total drawn current.

[0015]Further, preferably the charge manager controller comprises a
voltage reference, a current monitor coupled to the power source, and a
differential amplifier coupled to the voltage reference and the current
monitor. The voltage reference is proportional to the maximum current
limit, and the current monitor outputs an analog voltage that is
proportional to the total drawn current. The analog difference signal is
output by the differential amplifier and is proportional to the
difference between the voltage reference and the analog voltage.

[0016]In addition, preferably the charge manager and the power supply are
disposed within a common charger housing, and the charger housing
comprises a first indicator for indicating a charge state of the first
battery, and a second indicator for indicating a charge state of the
second battery.

[0017]As will be described in further detail below, the method of
simultaneously charging multiple batteries involves supplying a primary
charge current from a power source to a first of the batteries, while
charging a second of the batteries with a secondary charge current from
the power source in accordance with a continuous comparison between a
predefined maximum current limit and a total current drawn from the power
source.

[0018]In a preferred implementation, the second battery charging step
comprises controlling the magnitude of the secondary charge current by
outputting to a control input of a controlled current/voltage source an
analog difference signal that is proportional to the difference between
the magnitude of the maximum current limit and the total drawn current.

[0019]Further, the analog difference signal is proportional to the
difference between a voltage reference and an analog voltage signal.
Preferably, the voltage reference is proportional to the maximum current
limit, and the analog voltage signal is proportional to the total drawn
current.

[0020]In addition, preferably the first battery is disposed within a
handheld computing device, and the second battery is charged with a
continuously increasing portion of the secondary charge current while the
handheld computing device charges the first battery with a continuously
decreasing portion of the primary charge current. Preferably, the total
of the decreasing portion and the increasing portion is substantially
equal to the maximum current limit.

1. Battery Charger 100

[0021]Referring now to FIG. 1, there is shown a battery charger, denoted
generally as 100, according to the invention. The battery charger 100
includes a DC power source (not shown), and a charge manager 200.
Optionally, the battery charger 100 also includes a primary charge state
indicator 300.

[0022]The DC power source has a DC voltage output 104 for outputting a
substantially constant DC output voltage. Typically, the DC power source
comprises an AC/DC converter which is configured to convert an AC input
voltage to the DC output voltage. However, other DC power sources, such
as DC batteries and DC/DC converters, are also encompassed by the
invention.

[0023]The battery charger 100 also comprises a charger housing (not
shown). Preferably, both the DC power source and the charge manager 200
are disposed within the charger housing. The charger housing includes a
first battery interface for connection to a first battery 106, and a
second battery interface 108 for connection to a second battery 110.
Preferably, the charger housing also includes a recessed section for
retaining the second battery 110 in position, externally to the housing,
when the second battery 110 is connected to the second battery interface
108.

[0024]Further, preferably the first battery interface includes both a
first battery charge port 112, and a data port 114. The DC power source
is coupled to the first battery charge port 112 via the DC voltage output
104, and supplies primary charge current to the first battery 106 via
series resistors R25, R11 and the first battery charge port 112. As will
be explained below, the series resistors R25, R11 are low resistance
loads that are used to measure the total current that is drawn from the
DC power source.

[0025]As shown, preferably the first battery 106 is disposed within a
handheld computing device 150 which includes an internal battery charger
for charging the first battery 106. The handheld computing device 150
connects to the first battery charge port 112 through power lines having
sufficient gauge to carry the primary charge current to the handheld
computing device 150. The internal battery charger charges the first
battery 106 from the primary charge current that it receives from the
battery charger 100 via the power lines.

[0026]The handheld computing device 150 also interfaces with the data port
114 (if present) via data communication lines. Typically, the power lines
and the data communications lines are provided as a Universal Serial Bus
(USB) cable. In the example shown, the data communications lines comprise
USB D+/D- data lines. As will be explained, the battery charger 100 uses
data received from the handheld computing device 150, via the USB cable,
to determine when the handheld computing device 150 is connected to the
battery charger 100.

[0027]The charge manager 200 is connected at its input to the DC voltage
output 104 of the DC power source. The charge manager 200 is also
connected at its output to the second battery interface 108, and supplies
secondary charge current to the second battery 110 via the second battery
interface 108.

[0028]Typically, the total of the rated maximum charge current of the
first battery 106 and the rated maximum charge current of the second
battery 110 is greater than the maximum current limit of the power
source. Therefore, as will be explained in greater detail below, to avoid
the possibility of the power source being overloaded when both batteries
106, 110 are interfaced with the battery charger 100, the charge manager
200 is configured to charge the second battery 110 with a secondary
charge current whose magnitude is determined in accordance with a
continuous comparison between the maximum current limit and the total
current drawn from the power source.

[0029]The primary charge state indicator 300 (if present) indicates the
state of charge of the first battery 106. The primary charge state
indicator 300 is connected at its input to the DC voltage output 104 of
the DC power source. Typically, the primary charge state indicator 300 is
disposed within the charger housing, and includes a pair of
different-coloured indicator lamps which are visible from the outer
surface of the battery housing for providing a visual indication of the
charge state of the first battery 106.

[0031]As will be explained, the charge manager 200 is configured to
determine the available secondary charge current by continuously
comparing the total current that is drawn from the DC power source
against the maximum current limit of the DC power source. The charge
manager 200 ensures that the secondary charge current that is drawn by
the second battery 110 does not exceed the calculated available secondary
charge current.

2.1. Battery Charge Circuit 202

[0032]The battery charge circuit 202 includes a voltage supply input 208,
a battery output, a charge current program input 212, and a charge status
output pin 214. The battery charge circuit 202 is connected at its
voltage supply input 208 to the DC voltage output 104 of the DC power
source, and is connected at its battery output to the second battery
interface 108.

[0034]As will be explained, the battery charge circuit 202 supplies the
secondary charge current to the second battery 110, via both a constant
current charge mode, and a constant voltage charge mode.

2.1.1. Controlled Current/Voltage Source 216

[0035]The controlled current/voltage source 216 includes a voltage input,
a voltage/current output, and a control input 220 which controls the
voltage/current that is output by the controlled source 216 at the
current output. The controlled current/voltage source 216 is connected at
its voltage input to the voltage supply input 208, and is connected at
its voltage/current output to the battery output.

2.1.2. Source Controller 218

[0036]The source controller 218 includes a charge mode input, a battery
sensor input, a control output, and a charge status output.

[0037]The source controller 218 is connected at its charge mode input to
the charge current program input 212, and is connected at its battery
sensor input to the battery output. The source controller 212 is also
connected at its control output to the control input 220 of the
controlled source 216, and is connected at its charge status output to
the charge status output pin 214 of the battery charge circuit 202.

[0038]The source controller 218 includes control logic that controls the
signal that is output on the control output of the source controller. The
control logic is configured to provide the battery charge circuit 202
with two charge modes, based on the voltage at the battery output. In
constant current mode, the voltage/current output of the controlled
source 216 supplies a constant current to the battery output. In this
mode, the constant current is proportional to the resistance at the
charge current program input 212. In constant voltage mode, the
voltage/current output of the controlled source 216 maintains a constant
voltage at the battery output.

[0039]Typically, the battery charge circuit 202 transitions from constant
current mode to constant voltage mode when the voltage at the battery
output, as read by the source controller 218, exceeds a predetermined
float voltage.

[0040]The source controller 218 also includes logic circuitry that
indicates the charge state of the second battery 110. Typically, the
logic circuitry is configured to output a low impedance path to signal
ground at the charge status output when the voltage sensed at the battery
sensor input indicates that the second battery 110 is in constant-current
charge mode, and to output a larger impedance path to signal ground at
the charge status output when the voltage sensed at the battery sensor
input indicates that the second battery 110 is in constant-voltage charge
mode and the secondary charge current has dropped to less than 10% of the
constant charge current. Therefore, the charge status output will have a
low impedance when the second battery 110 is in constant-current charge
mode, and will have a high impedance when the second battery 110 is being
trickle charged in constant-voltage charge mode.

2.2. Charge Controller 204

[0041]The charge controller 204 comprises a DC voltage reference 222, a
load current monitoring circuit 224, and a differential amplifier 226
that is coupled to the DC voltage reference 222 and the load current
monitoring circuit 224.

[0042]As will be explained below, the charge controller 204 is configured
to control the magnitude of the secondary charge current that is output
by the battery charge circuit 202. To do so, the charge controller 204
outputs to the control input of the battery charge circuit 202 an analog
difference signal that is proportional to the difference between the
magnitude of the maximum current limit of the DC power source and the
total current that is drawn from the DC power source.

2.2.1. Load Current Monitoring Circuit 224

[0043]The load current monitoring circuit 224 comprises a current monitor
228, and a load resistor 230.

[0044]The current monitor 228 includes a voltage sense input 232, and a
current sense output 234. The current monitor 228 is connected at its
voltage sense input 232 across the series resistors R25, R11, and is
connected at its current sense output 234 to the load resistor 230.

[0045]The current monitor 228 measures the voltage drop at the voltage
sense input 232, and outputs a current at the current sense output 234.
The magnitude of the current output at the current sense output 234 is
proportional to the voltage drop measured at the voltage sense input 232.
Since the voltage drop measured at the voltage sense input 232 is
proportional to the current drawn from the DC power source, the resulting
analog voltage that is developed across the load resistor 230 is
proportional to the magnitude of the current that is drawn from the DC
power source.

2.2.2. Differential Amplifier 226

[0046]The differential amplifier 226 includes an inverting input, a
non-inverting input, and a difference output. The differential amplifier
226 is connected at its non-inverting input to the DC voltage reference
222, and is connected at its inverting input to the junction of the load
resistor 230 and the current sense output 234. Further, the differential
amplifier 226 is connected at its difference output to the charge current
program input 212 of the battery charger 202.

[0047]The differential amplifier 226 is configured to output an analog
difference signal (via the difference output) that is proportional to the
difference between the DC voltage reference 222 and the analog voltage
that is output by the current monitor 228. Further, the magnitude of the
voltage at the DC voltage reference 222 is proportional to the maximum
current limit of the DC power source. Therefore, the analog difference
signal that is output by the differential amplifier 226 is proportional
to the difference between the maximum current limit of the DC power
source and the total current that is drawn from the DC power source.

[0048]As discussed above, in constant current mode, the magnitude of the
current that is output by the battery charge circuit 202 is proportional
to the resistance at the charge current program input 212. Therefore, in
constant current mode, the magnitude of the secondary charge current that
is supplied to the second battery 110 is proportional to the current that
is available from the DC power source. Preferably, the magnitude of the
secondary charge current in constant current mode is equal to the current
that is available from the DC power source.

2.3. Secondary Charge State Indicator 206

[0049]The secondary charge state indicator 206 indicates the charge state
of the second battery 110.

[0050]As shown, preferably the secondary charge state indicator 206
provides a visual indication of the battery charge state, and comprises a
red LED, and a green LED and suitable lamp driver circuitry. The lamp
driver circuitry provides drive current to the LEDs, and includes a
control input that is connected to the charge status output pin 214 of
the battery charge circuit 202. Preferably, the lamp driver circuitry
lights the red LED when the charge status output pin 214 indicates that
the second battery 110 is in constant-current charge mode, and lights the
green LED when the charge status output pin 214 indicates that the second
battery 110 is trickled charged in constant voltage charge mode.

[0052]As will be explained below, the primary charge state indicator 300
is configured to indicate the charge state of the first battery 106 based
on the magnitude of the current that is drawn by the first battery 106.

3.1. Charge State Detection Circuit 302

[0053]The charge state detection circuit 302 comprises a DC voltage
reference 322, a load current monitoring circuit 324, and a comparator
326 that is coupled to the DC voltage reference 322 and the load current
monitoring circuit 324.

[0054]As will be explained below, the charge state detection circuit 302
is configured to monitor the magnitude of the primary charge current that
is drawn from the first battery charge port 112. To do so, the charge
state detection circuit 302 outputs to a control input of the primary
charge state indication circuit 306 an analog difference signal that is
proportional to the difference between the magnitude of the rated
constant charge current for the first battery 106 (for constant-current
charge mode) and the current that is drawn from the battery charger 100
via the first battery charge port 112.

3.1.1. Load Current Monitoring Circuit 324

[0055]The load current monitoring circuit 324 comprises a current monitor
328, and a load resistor 330.

[0056]The current monitor 328 includes a voltage sense input 332, and a
current sense output 334. The current monitor 328 is connected at its
voltage sense input 332 across the series resistor R11, and is connected
at its current sense output 334 to the load resistor 330.

[0057]The current monitor 328 measures the voltage drop at the voltage
sense input 332, and outputs a current at the current sense output 334.
The magnitude of the current output at the current sense output 334 is
proportional to the voltage drop measured at the voltage sense input 332.
Since the voltage drop measured at the voltage sense input 332 is
proportional to the current that is drawn from the DC power source via
the first battery charge port 112, the resulting analog voltage that is
developed across the load resistor 330 is proportional to the magnitude
of the current that is drawn by the first battery 106 and the handheld
computing device 150.

3.1.2. Comparator 326

[0058]The comparator 326 includes an inverting input, a non-inverting
input, and a difference signal output. The comparator 326 is connected at
its non-inverting input to the DC voltage reference 322, and is connected
at its inverting input to the junction of the load resistor 330 and the
current sense output 334. Further, the comparator 326 is connected at its
signal output to a control input 336 of the primary charge state
indication circuit 306.

[0059]The comparator 326 is configured to output to the primary charge
state indication circuit 306 (via the signal output) an analog signal
whose magnitude is based on the greater of the magnitude of the DC
voltage reference 322 and the analog voltage that is output by the
current monitor 328. Further, preferably the magnitude of the voltage at
the DC voltage reference 322 is less than the analog voltage that is
output by the current monitor 328 when the first battery 106 is in
constant-current charge mode, and is greater than the maximum analog
voltage that is output by the current monitor 328 when the first battery
106 is in constant-voltage charge mode and the primary charge current has
dropped to less than 10% of the constant charge current. Therefore, the
output signal of the comparator 326 will have a low voltage when the
first battery 106 is in constant-current charge mode, and will have a
high voltage when the first battery 106 is being trickle charged in
constant-voltage charge mode.

3.2. Primary Charge State Indication Circuit 306

[0060]The primary charge state indication circuit 306 indicates the charge
state of the first battery 106.

[0061]As shown, preferably the primary charge state indication circuit 306
provides a visual indication of the battery charge state, and comprises a
red LED, a green LED and suitable lamp driver circuitry. The lamp driver
circuitry provides drive current to the LEDs, and includes a control
input 336 that is connected to the signal output of the comparator 326 of
the charge state detection circuit 302. Preferably, the lamp driver
circuitry lights the red LED when the signal output of the comparator 326
indicates that the first battery 106 is being charged with a constant
current, and lights the green LED when the signal output of the
comparator 326 indicates that the first battery 106 is being trickle
charged in constant-voltage charge mode.

[0062]Preferably, the lamp driver circuitry also includes a gate input 338
that is used to enable/disable the charge indication produced by the
primary charge state indication circuit 306.

[0064]The differential amplifier 340 includes an inverting input, a
non-inverting input, and a difference signal output. Pullup resistor R22
is connected between the first battery charge port 112 and the inverting
input of the differential amplifier 340. Pulldown resistor R27 is
connected between the inverting input of the differential amplifier 340
and ground. Similarly, pullup resistor R13 is connected between the first
battery charge port 112 and the non-inverting input of the differential
amplifier 340. Pulldown resistor R23 is connected between the
non-inverting input of the differential amplifier 340 and ground.

[0065]The differential amplifier 340 is also connected at its
non-inverting input to the D- data line of the data port 114, and is
connected at its inverting input to the D+ data line of the data port
114. Further, the differential amplifier 340 is connected at its signal
output to the gate input 338 of the primary charge state indication
circuit 306.

[0066]The differential amplifier 340 is configured to output to the charge
state detection circuit 302 (via the difference signal output) an analog
signal whose magnitude is based on the greater of the magnitude of the
voltages that are present at its respective inputs. If no handheld
communications device 150 is connected to the data port 114, the
pullup/pulldown resistors R13, R22, R23, R27 cause the differential
amplifier 340 to output a high voltage to the gate input 338 of the
primary charge state indication circuit 306. In this state, the primary
charge state indication circuit 306 is disabled, and neither of its LEDs
is turned on.

[0067]On the other hand, if a high speed handheld communications device
150 is connected to the data port 114 via a USB cable, the device 150
pulls the D+ data line high (thereby indicating that it is a high speed
device). Since the voltage at the non-inverting input to the differential
amplifier 340 will be greater than the voltage at the inverting input,
the differential amplifier 340 will output a low voltage to the gate
input 338 of the primary charge state indication circuit 306. In this
state, the primary charge state indication circuit 306 is enabled,
thereby allowing the primary charge state indication circuit 306 to
activate the appropriate one of its LEDs based on the charge state of the
first battery 106.

Method of Operation

[0068]The charging method effected by the battery charger 100 will now be
described, by way of overview, with reference to FIG. 2.

[0069]Initially, at step S100, the batteries 106, 110 are electrically
interfaced with the battery charger 100, and the DC power source in the
battery charger 100 is connected to an AC power source.

[0070]At step S102, the battery charger 100 begins to supply charge
current to the first battery 106 from the DC power source. Concurrently,
the charge manager 200 continuously monitors the total current that is
drawn from the DC power source, and supplies charge current to the second
battery 110, from the DC power source, based on a continuous comparison
between a predefined maximum current limit and the total current that is
drawn from the DC power source.

[0071]An advantageous feature of the battery charger 100 is that it allows
the first battery 106 to be charged with a first continuously decreasing
portion of the maximum current that is available from the DC power
source, while simultaneously charging the second battery 110 with a
continuously increasing portion of the available current. In this phase,
the total of the decreasing portion and the increasing portion is
substantially equal to the maximum available current. As a result, more
efficient use is made of the current capacity of the DC power source,
thereby reducing total charging time for the batteries 106, 110.

[0072]Another advantageous feature of the battery charger 100 is that the
predefined maximum current limit can be varied to apportion charge
current between the batteries 106, 110 as desired. For instance, if the
predefined maximum current limit is set equal to the maximum current
limit of the DC power source, the first battery 106 will have full charge
priority over the second battery 110. As a result, the first battery 106
will receive its rated charge current (assuming that the rated charge
current is less than the maximum current limit of the DC power source),
and the second battery 110 will be charged with the remaining (if any)
current of the DC power source.

[0073]However, if the predefined maximum current limit is set greater than
the maximum current limit of the DC power source, the first battery 106
will receive a charge current which is less than its rated charge
current, depending upon the maximum current limit of the DC power source.
Simultaneously, the second battery 110 will be charged with a charge
current which is greater than that for the variation where the first
battery 106 receives its rated charge current.

[0074]The operation of the battery charger 100 will now be explained in
greater detail with reference to FIGS. 3A, 3B and 4.

[0075]At the commencement of a charge operation, at step S200, the first
battery 106 is disposed within the handheld computing device 150, and the
device 150 is connected to the first battery charge port 112, typically
via a USB cable. Further, the second battery 110 is retained within the
recessed portion of the charger housing, and is connected to the second
battery interface 108.

[0076]The battery charger 100 is then connected to a source of AC power,
at step S202, thereby supplying DC power to the first battery 106 and the
second battery 110. Upon application of the DC power, the handheld
computing device 150 enters the first master charge mode, and the charge
manager 200 enters the first slave charge mode.

[0077]In first master charge mode, the handheld computing device 150
charges the first battery 106 with a substantially constant charge
current. Typically, this charge current constitutes a major portion of
the current that is available from the DC power source. Further, as will
be discussed below, the current that can be drawn from the DC power
source can be apportioned between the first battery 106 and the second
battery 110 through the appropriate selection of the resistance of the
load resistor 230 and the magnitude of voltage produced by the DC voltage
reference 222. Therefore, depending upon the maximum current limit of the
DC power source, and the manner in which the charge current is
apportioned between the batteries 106, 110, the magnitude of the constant
charge current applied to the first battery 106 in the first master
charge mode may be equal to or less than the rated constant-current
charge current for the first battery 106.

[0078]The charge state detection circuit 302 continuously monitors the
current that is drawn from the DC power source via the first battery
charge port 112. The DC voltage reference 322 and the load resistor 330
are selected such that, when the handheld computing device 150 charges
the first battery 106 with a major portion of the current available from
the DC power source, the comparator 326 of the charge state detection
circuit 302 outputs a low voltage output signal. Since the control input
336 of the primary charge state indication circuit 306 is connected to
the signal output of the comparator 326, the low voltage output signal in
first master charge mode causes the primary charge state indication
circuit 306 to light its red LED, thereby indicating that the first
battery 106 is in constant-current charge mode.

[0079]Concurrently, the charge manager 200 continuously monitors the total
current that is drawn from the DC power source, and thereby continuously
determines the magnitude of additional current (if any) that is available
to be drawn from the DC power source. The load resistor 230 and the DC
voltage reference 222 are selected such that, when the handheld computing
device 150 charges the first battery 106, the current drawn by the charge
manager 200 and the second battery 110 from the DC power source does not
exceed the remaining portion of the current that is available from the DC
power source.

[0080]Therefore, in first slave charge mode, the charge manager 200
charges the second battery 110 with a substantially constant charge
current. Depending upon the magnitude of the charge current that is
available from the DC power source, and the rated charge current of the
second battery 110, the total of the charge current applied to the first
battery 106 and the charge current drawn by the charge manager 200 and
the second battery 110 is typically substantially equal to, or at least
does not exceed, the maximum current limit of the DC power source.
Further, depending upon the maximum current limit of the DC power source,
and the magnitude of the current that is drawn by the first battery 106,
the magnitude of the constant charge current applied to the second
battery 110 in the first slave charge mode is typically less than the
rated charge current for the second battery 110 and constitutes a minor
portion of the maximum current limit of the DC power source.

[0081]The logic circuitry of the source controller 218 also continuously
monitors the voltage at the battery sensor input. Since voltage at the
battery sensor input is usually less than the float voltage of the second
battery 110 when the second battery is in constant-current charge mode,
the logic circuitry outputs a low impedance path to signal ground at the
charge status output. Further, since the control input of the secondary
charge state indicator 206 is connected to the charge status output pin
214 of the battery charge circuit 202, the low impedance signal in first
slave charge mode causes the secondary charge state indicator 206 to
light its red LED, thereby indicating that the second battery 110 is in
constant-current charge mode.

[0082]As shown by step S204, the handheld computing device 150 maintains
the first master charge mode until the voltage of the first battery 106
reaches the rated float voltage for the first battery 106. Thereafter, at
step S206, the handheld computing device 150 exits the first master
charge mode and enters the second master charge mode.

[0083]In the second master charge mode, the handheld computing device 150
charges the first battery 106 with a substantially constant voltage. Due
to the capacitance of the first battery 106, the magnitude of the charge
current to the first battery 106 in the second master charge mode
decreases exponentially.

[0084]The charge manager 200 continues to determine the magnitude of
additional current that is available to be drawn from the DC power
source. Therefore, the charge manager 200 continues to charge the second
battery 110 with the remaining portion of the available current (if any)
until the handheld computing device 150 enters the second master charge
mode. At this point, the charge manager 200 exits the first slave charge
mode and enters the second slave charge mode.

[0085]As the magnitude of the current drawn by the handheld computing
device 150 from the DC power source diminishes exponentially, the
magnitude of the analog difference signal that is output by the
differential amplifier 226 diminishes. Since the difference output of the
differential amplifier 226 is coupled to the charge current program input
212 of the battery charger 202, at step S208 the diminishing analog
difference signal in second slave charge mode causes the secondary charge
current applied to the second battery 110 (and hence the current drawn by
the battery charge circuit 202 and the second battery 110) to increase
exponentially.

[0086]Due to the negative feedback loop defined by the battery charge
circuit 202 and the load current monitoring circuit 224, the diminishing
analog difference signal also causes the magnitude of the current drawn
by the battery charge circuit 202 and the second battery 110 in second
slave charge mode to be substantially equal to the remaining portion of
the current that is available from the DC power source. As a result, the
total current that is drawn from the DC power source remains
substantially constant.

[0087]As shown by step S210, the charge manager 200 continues to increase
the charge current to the second battery 110 until the secondary charge
current equals the rated constant-current charge current for the second
battery 110. At this point, the charge manager 200 enters the third slave
charge mode, at step S212, and charges the second battery 110 with a
constant current which is substantially equal in magnitude to the rated
charge current. Typically, this charge current constitutes a major
portion of the current available from the DC power source.

[0088]Concurrently, the handheld computing device 150 continues to charge
the first battery 106 with a substantially constant voltage. In view of
the limited current available form the DC power source, and the current
drawn by the second battery 110, the magnitude of the charge current
applied to the first battery 106 in the second master charge mode
continues to be a minor portion of the current available from the DC
power source. As a result, the current drawn by the charge manager 200
from the DC power source in the second master charge mode does not exceed
the remaining portion of the current that is available from the DC power
source.

[0089]The charge state detection circuit 302 continues to monitor the
current that is drawn from the DC power source via the first battery
charge port 112. The DC voltage reference 322 and the load resistor 330
are selected such that, when the primary charge current has dropped to
less than 10% of the constant charge current, the comparator 326 of the
charge state detection circuit 302 outputs a higher voltage output
signal. Since the control input 336 of the primary charge state
indication circuit 306 is connected to the signal output of the
comparator 326, the higher voltage output signal in second master charge
mode causes the primary charge state indication circuit 306 to extinguish
its red LED and to light its green LED, thereby indicating that the first
battery 106 is being trickle charged.

[0090]As shown by step S214, the charge manager 200 continues to charge
the second battery 110 with the rated constant charge current until the
voltage measured at the charge mode input of the source controller 218
reaches the rated float voltage for the second battery 110. Thereafter,
at step S216, the charge manager 200 exits the third slave charge mode
and enters the fourth slave charge mode.

[0091]In the fourth slave charge mode, the charge manager 200 charges the
second battery 110 with a substantially constant voltage. Due to the
capacitance of the second battery 110, the magnitude of the charge
current in the fourth slave charge mode decreases exponentially.

[0092]The source controller 218 continuously monitors the current that is
drawn by the second battery 110. When the secondary charge current has
dropped to less than 10% of the constant charge current, the logic
circuitry of the source controller 218 outputs a higher impedance path to
signal ground at the charge status output. Since the control input of the
secondary charge state indicator 206 is connected to the charge status
output pin 214 of the battery charge circuit 202, the higher impedance
signal in fourth slave charge mode causes the secondary charge state
indicator 206 to extinguish its red LED and to light its green LED,
thereby indicating that the second battery 110 is being trickle charged.

[0093]The charge profiles for the batteries 106, 110 are shown in FIG. 4.

Exemplary Implementation of Battery Charger 100

[0094]FIG. 5 depicts an exemplary implementation of a battery charger
100'. Apart from the specific implementation details, the battery charger
100' is substantially identical to the battery charger 100. For ease of
understanding, FIG. 5 uses similar reference numerals as in FIG. 1, but
denoted with a prime superscript to refer to the corresponding elements
of FIG. 1.

[0098]The differential amplifier 226 of the charge controller 204 is
implemented with a Texas Instruments TLV2711 Operational Amplifier. The
comparator 326 of the charge state detection circuit 302 is implemented
with a Texas Instruments LMV339 Low-Voltage Comparator.

[0099]The current monitors 228, 328 are each implemented with a Zetex
Semiconductors ZXCT1009 High-Side Current Monitor integrated circuit.

[0100]The load resistor 230 of the load current monitoring circuit 224 is
implemented with resistor R26 in series with resistor R6. The load
resistor 330 of the load current monitoring circuit 324 is implemented
with resistor R14.

[0103]The battery charger 100' includes operation amplifier U1B, which is
implemented with a Texas Instruments TLV2711 Operational Amplifier. The
inverting input terminal of the operation amplifier U1B is connected to
the junction of the resistors R26, R6 of the load current monitoring
circuit 224. The signal output of operation amplifier U1B is connected to
a shutdown input of the DC power supply, and serves to disable the DC
power supply if the current drawn from the DC power supply, as measured
by the current monitor 228, exceeds the rated maximum current for the
supply.

[0104]The battery charger 100' also includes a battery temperature monitor
circuit, which is implemented with comparators U2B, U2D, and ladder
resistors R4, R15, R5. The comparators U2B, U2D are implemented with a
Texas Instruments LMV339 Low-Voltage Comparator. The inverting input of
comparator U2B and the non-inverting input of comparator U2D are commonly
connected to a thermistor in the second battery 110. The difference
outputs of the comparators U2B, U2D, which are commonly connected to the
gate input of transistor Q3a, serve to disable the battery charge circuit
202' when the temperature of the second battery 110 exceeds a rated
maximum.

[0105]When the handheld computing device 150 is connected to the battery
charger 100, the DC power source (via VBUSin) supplies the handheld
computing device 150 with primary charge current via the VBUS. The
handheld computing device 150 applies a constant charge current to the
first lithium battery 106. As this point, the voltage at the
non-inverting input to the comparator U2c will be greater than 1.8 VDC,
thereby causing the red LED D5 to turn on. In this mode, typically the
handheld computing device 150 charges the first battery 106 with a major
portion of the current that is available from the DC power source.

[0106]At the same time, the operation amplifier U1A adjusts the resistance
at the PROG pin of the charge circuit 202' (via transistors Q3a, Q3b and
resistor R8) such that the charge circuit 202' charges the second lithium
battery 110 with the remaining portion of the current that is available
from the DC power source. As this point, the CHARG_N output of the charge
circuit 202' is pulled low, thereby causing the red LED D2 to turn on.

[0107]The handheld computing device 150 continues to charge the first
battery 106 with the constant charge current until the voltage of the
battery 106 reaches its rated voltage (4.2 V DC). Thereafter, the
handheld computing device 150 charges the first battery 106 with a
constant voltage (approx. 4.2 V DC). During this mode, the primary charge
current drawn by the handheld computing device 150 from the VBUS
decreases exponentially.

[0108]In response to the exponentially-decreasing primary charge current,
the operation amplifier U1A continues to decrease the resistance at the
PROG pin of the charge circuit 202' such that the charge circuit 202'
charges the second lithium battery 110 with an exponentially increasing
secondary charge current. The feedback loop defined by the transistors
Q3a, Q3b, the charge circuit 202', and the current monitor 228 maintains
the total of the primary charge current and the secondary charge current
substantially constant.

[0109]When the primary charge current drops below a predetermined minimum
value (e.g. 10% of rated constant-charge current), the voltage at the
non-inverting input to the comparator U2c will be less than 1.8 v,
thereby causing the red LED D5 to turn off and the green LED D1 to turn
on.

[0110]When the secondary charge current reaches the rated charge current
for the second battery 110, the charge manager 200' charges the second
battery 110 with a constant major portion of the current that is
available from the DC power source. The feedback loop defined by the
transistors Q3a, Q3b, the charge circuit 202', and the current monitor
228 prevents the secondary charge current from exceeding the charge
current that is available from the DC power source.

[0111]The charge manager 200' continue to charge the second battery 110
with the constant charge current until the voltage of the battery 110
reaches its rated voltage (4.2 V DC). Thereafter, the charge manager 200'
charges the battery 110 with a constant voltage (approx. 4.2 V DC).
During this mode, the secondary charge current drawn by the battery 110
decreases exponentially.

[0112]When the secondary charge current drops below a predetermined
minimum value (e.g. 10% of rated constant-charge current), the CHARG_N
output of the charge circuit 202' is pulled weakly low, thereby causing
the red LED D2 to turn off and the green LED D4 to turn on.

[0113]In the preceding example, the first battery 106 was authorized to
draw up its rated charge current, and the battery charger 100 would
charge the second battery 110 with all of the remaining available charge
current. To implement this solution, the magnitude of the DC voltage
reference 222 of the charge controller 204 would be equal to the voltage
that would be dropped across the load resistor 230 of the load current
monitoring circuit 224 when the total current drawn from the DC power
source was at the maximum current limit for the power source. With this
implementation, the following two charging scenarios are possible (each
assuming that the maximum current limit of the DC power source was 1.0
A):

[0114]1. If the rated charged current for the second battery 110 was 0.5
A, both batteries 106, 110 would charge at their full rates.

[0115]2. If rated charged current for the first battery 106 was 0.7 A, and
the rated charged current for the second battery 110 was 0.5 A, the first
battery 106 would still charge at its full rate. Since the remaining
available charge current would be 0.3 A, the second battery 110 would
charge at 60% (0.3 A/0.5 A) of its full rate.

[0116]However, the magnitude of the DC voltage reference 222 can be
adjusted so that the initial maximum current limit used by the battery
charge manager 200 is other than the maximum current limit of the DC
power source. By doing so, the initial current split between the first
battery 106 and the second battery 110 can be varied as desired. For
instance, the voltage output by the DC voltage reference 222 could be
varied from the above value to thereby limit the maximum current that is
drawn by the first battery 106 to 90% of the maximum current limit of the
DC power source. With this latter implementation, the following
additional charging scenario would be possible (again assuming that the
maximum current limit of the DC power source was 1.0 A):

[0117]3. If the rated charged current for the first battery 106 was 1.0 A,
and the rated charged current for the second battery 110 was 1.0 A, the
first battery 106 would charge at 90% (0.9 A) of its full rate, and the
second battery 110 would charge at 10% (0.1 A) of its full rate.

[0118]Further, the voltage output by the DC voltage reference 222 could be
varied to give charge priority to the second battery 110, as opposed to
the first battery 106. It will be appreciated, therefore, that the
foregoing embodiment offers considerable flexibility in terms of the
possible charging scenarios, in contrast to the prior art.

[0119]The scope of the monopoly desired for the invention is defined by
the claims appended hereto, with the foregoing description being merely
illustrative of the preferred embodiment of the invention. Persons of
ordinary skill may envisage modifications to the described embodiment
which, although not explicitly suggested herein, do not depart from the
scope of the invention, as defined by the appended claims.

Patent applications by Dusan Veselic, Oakville CA

Patent applications in class With detection of current or voltage differential (e.g., slope, etc.)

Patent applications in all subclasses With detection of current or voltage differential (e.g., slope, etc.)